As shown in Japanese Laid-Open No. 2000-356441, for example, there is a well-known auger type ice making machine provided with a freezing cylinder which has an evaporator on its outer peripheral surface and into which water used for making ice is supplied, wherein the freezing cylinder is cooled by a freezing apparatus in which refrigerant discharged from a compressor driven by a motor is circulated through a condenser, a dryer and the evaporator to form ice on the inner surface of the freezing cylinder, the thus-formed ice being scraped off and extruded by an ice-scraping auger driven by an auger motor. Such auger type ice making machine has a thermal expansion valve on the upstream side of the evaporator. The thermal expansion valve is designed to open more, with increase in temperature of the refrigerant in the downstream side of the evaporator, to control the amount of refrigerant flowing into the evaporator on the basis of the refrigerant temperature at the evaporator outlet, thereby securing specified performance in making ice.
In the above scheme where the refrigerant flow is controlled on the basis of the refrigerant temperature at the evaporator outlet, when ambient temperature or water temperature is high, the performance of the freezing apparatus (particularly, the compressor) decreases. Since a significant heat load is applied to the freezing cylinder, in addition, the refrigerant pressure of the downstream side of the thermal expansion valve increases, and so does the evaporative temperature of the refrigerant in the evaporator. Although water temperature in the freezing cylinder is near 0° C. during stable operation, relatively high evaporative temperature of the refrigerant and water temperature cause decreased amount of heat exchange of the freezing cylinder, resulting in decreased amount of ice manufactured per unit of time. When ambient temperature or water temperature is low, on the other hand, the performance of the freezing apparatus (particularly, the compressor) increases. Since the heat load applied to the freezing cylinder decreases, in addition, the refrigerant pressure of the downstream side of the thermal expansion valve decreases, and so does the evaporative temperature of the refrigerant in the evaporator. In this case, relatively low evaporative temperature of the refrigerant and water temperature cause increased amount of heat exchange of the freezing cylinder, resulting in increased amount of ice manufactured per unit of time.
Such conventional auger type ice making machine which uses the thermal expansion valve to control the refrigerant flow on the basis of the refrigerant temperature at the evaporator outlet causes a problem that the auger type ice making machine is failure-prone due to the following reasons: the auger type ice making machine designed to have sufficient performance in making ice at high ambient temperature and water temperature has excessive performance in making ice at low ambient temperature and water temperature, resulting in heavy load applied to an auger motor for driving an ice-scraping auger, significant thrust exerted on the blade of the ice-scraping auger at the scraping of ice formed on the inner surface of the freezing cylinder, and ice clogging caused by increased resistance applied to the blade of the auger at the passage of ice.
In addition to the above-described scheme, there is another well-known scheme in which a constant pressure expansion valve that keeps refrigerant on the output side at a constant pressure is provided on the upstream side of the evaporator, thereby controlling the refrigerant flow on the basis of the refrigerant pressure at the evaporator inlet. In this scheme, when ambient temperature or water temperature is high, the performance of the freezing apparatus (particularly, the compressor) decreases. Since a significant heat load is applied to the freezing cylinder, in addition, the refrigerant pressure at the evaporator inlet (downstream side of the constant pressure expansion valve) increases, and so does the evaporative temperature of the refrigerant. Because the constant pressure expansion valve is designed to maintain the pressure of the downstream side thereof, the amount of the refrigerant to be supplied to the evaporator is reduced. As a result, a phenomenon in which liquid refrigerant does not reach to the evaporator outlet occurs, hindering the function of the freezing cylinder to decrease the performance in making ice. When ambient temperature or water temperature is low, on the other hand, the performance of the freezing apparatus (particularly, the compressor) increases. Since the heat load applied to the freezing cylinder decreases, in addition, the refrigerant pressure at the evaporator inlet (downstream side of the constant pressure expansion valve) decreases, and so does the evaporative temperature of the refrigerant. Because the constant pressure expansion valve is designed to maintain the pressure of the downstream side thereof, the amount of the refrigerant to be supplied to the evaporator increases. As a result, there is a phenomenon in which even though liquid refrigerant has reached to the evaporator outlet, the constant pressure expansion valve keeps supplying refrigerant, resulting in refrigerant liquid back to the compressor.
In such conventional auger type ice making machine which uses the constant pressure expansion valve to control the refrigerant flow on the basis of the refrigerant pressure at the evaporator inlet, the constant pressure value of the constant pressure expansion valve is determined in consideration of the difference between the evaporative temperature of refrigerant and the temperature of the freezing cylinder as well as the balance between the range to which liquid refrigerant reaches and refrigerant liquid back to the compressor. As described above, when ambient temperature or water temperature is low, however, such freezing apparatus using the constant pressure expansion valve tends to present a problem of refrigerant liquid back to the compressor. In addition, such freezing apparatus also causes a problem that sufficient performance in making ice cannot be delivered when ice grows in demand, i.e., when ambient temperature or water temperature is high.